Database Reference
In-Depth Information
Local response time
Service time
Queuing time
CPU queuing
CPU time
QU BE: 0
Disk queuing
Disk service time
Synchronous read
Synchronous write (log)
Asynchronous read
(non-overlapped part)
Lock wait
QU BE: 0
Other waits
QU BE: 0
Figure 5.1 Local response time is the sum of the service time and the queuing time.
Queuing Time
In a normal multiuser environment, concurrent programs cause many kinds of
contention for the resources required; they will therefore have to queue for these
resources. The following resources are within the domain of the LRT:
ž CPU queuing time (all processors are busy with high-priority tasks).
ž Disk drive queuing (the drive containing the requested page is busy).
ž Lock waits (the requested table page or row is locked at an incompati-
ble level).
ž Multiprogramming levels, number of threads, or other throttles have
reached their limits (these are system values designed to prevent resource
overload situations; it is not necessary to understand these for the purpose
of this topic).
With the exception of disk drive queuing, the QUBE ignores queuing in order
to provide a simple estimation procedure for the areas that will normally have far
greater performance implications. We will discuss this area, together with some
other simplifying assumptions behind the QUBE formula in more detail later.
The result is a very simple estimation process with merely two input vari-
ables, TR and TS, with a third variable, F, when necessary. These are able to
take into consideration the processing and I/O costs of the SQL, and are what pri-
marily influence the index design process. They are shown in Figure 5.2 together
with the values used in the QUBE. If the QUBE is being used to compare the
performance of alternative access paths (Comparison, in Figure 5.2), the number
of FETCH calls will be constant and therefore can be ignored. If it is being used
to determine the response time (Absolute), then they would be included. For the
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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